To get high quality reception, communication systems, which include radio and television, require a strong signal that is not corrupted by noise or interference. One form of interference that can severely degrade reception is multipath. Multipath occurs when the transmitted signal arrives at the receiver simultaneously from more than one direction. The multiple paths are generally due to reflections of the transmitted signal from hills, buildings, etc.; they can also be the result of atmospheric phenomena. The indirect paths are longer than the direct path, and consequently, the indirect path signals arrive at the receiver later in time than the corresponding direct path signal. This makes them arrive at the receiver with a different phase than the direct path signal, and, consequently, causes distortion in both the phase and the amplitude of the received signal. This can result in deep signal strength fades, overlapping data, clicking, etc. Examples of multipath distortion are ghosts on TV, degraded fidelity in commercial FM stereo, and loss of data in communication links.
Designing the antenna pattern gain characteristics to reject the indirect paths by placing a null in their direction of arrival is one of the better approaches to reducing multipath distortion. This eliminates the indirect paths altogether. It is easy to accomplish when conditions are known and do not change. But in most communication situations, conditions do change. The adaptive array has been used to automatically change the antenna pattern as the conditions change.
In applying an adaptive array to the general communications problem where the direction of arrival (DOA) and the time of arrival (TOA) of the signal of interest are unknown, the least means squared error algorithm (LMS) is well suited. For optimal results, the LMS adaptive array requires a reference signal which is a replica of the signal of interest.
Generation of the reference signal can pose a problem. In practice, a replica of the transmitted signal is not available at the receiver. The reference signal must be derived from the adaptive array output signal. Robert Riegler and Ralph Compton (Proceedings of the IEEE, Vol. 61, No. 6, June 1973, p. 748) have discussed the application of the adaptive array to amplitude modulated communications signals, where the adaptive array output signal is processed to generate a representation of the carrier of the transmitted signal for use as the reference signal. But this approach addresses interference signals, not the multipath problem.
R. T. Compton and D. M. DiCarlo (IEEE Transactions on Aerospace and Electronic Systems, VOL. AES-14, NO. 4, July 1978, p. 599) and Y. Bar-Ness (IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-18, No. 1, January 1982, p. 115) analyze another adaptive array which uses the array output to generate the reference signal. But their system was designed to address a signal environment in which the signal of interest is received along with a wideband interference signal. They do not address the multipath problem.
Ralph Compton (Proceedings of the IEEE, Vol. 66, No. 3, March 1978, p. 289) discusses an adaptive array for communication signals using a spread spectrum technique. The adaptive array uses knowledge of the spreading code to generate a reference signal. August McGuffin (U.S. Pat. No. 4,217,586) has extended this approach by utilizing the multipath in the reference signal generation. The pseudo random (PN) code based reference signal generator can keep lock even in severe multipath fading. But both these approaches require a known PN code be present in the transmitted signal to generate a reference signal.
G. H. Persinger (1977 International Conference on Communications, IEEE, Pt. III, Chicago, Ill., 12-15 June, 1977, Pp. 259-262) has used a low level PN code placed in quadrature (90 degrees out of phase) with a transmitted AM signal. It is used to generate the reference signal at the receiver. The reference generation is dependent on the injection of this special signal with a known code.
Peder Hansen (IEEE Transactions on Antennas and Propagation, Vol. AP-29, No. 6 November 1981, p. 836) has placed a special modulated pilot signal in the transmitted signal to be used to generate the reference signal. This technique was used specifically to discriminate against multipath. But it does not work without the special pilot signal.
Gayle Martin (U.S. Pat. No. 4,255,791) uses noise decorrelation to generate a reference signal for an adaptive array. This method addresses an environment where there is a large interfering signal, not the multipath environment.
Kenneth F. Rilling in U.S. patent application Ser. No. 819,416, filed on Jan. 16, 1986, entitled Anti-multipath Signal Processor, has amplitude limited the adaptive array output signal to generate the reference signal. This system rejects unwanted multipath and low level noise. But this work is limited to a reference signal implementation.
In a related technology, transversal filters (single input adaptive filters) which reduce TV ghosts by signal processing (not by using the antenna pattern) use the known portions of the transmitted TV signal structure to generate the reference signal (Shri Goyal, others, IEEE Transactions on Consumer Electronics, Vol. CE-26, February 1980). Transversal filters remove the ghosts after the received signal has been demodulated. But, they require a large number of loops, and they are generally microprocessor or computer based. Consequently, they are quite complicated and expensive.
An alternative to deriving the reference signal, is the elimination of the reference signal altogether by changing the feedback equations. Work along this line has been performed by John Treichler in a related technology with a single input adaptive filter for constant modulus (amplitude) signals (John R. Treichler and Brian G. Agee, IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-31, No. 2, 1983, P. 459; M. G. Larimore and J. R. Treichler, International Conference of Acoustics, Speech, and Signal Processing 1983, Boston, P. 13). The Constant Modulus Algorithm (CMA) can be used to remove unwanted multipath for constant amplitude signals because it exploits the amplitude fluctuations induced by multipath. The CMA approach has limitations: (1) It only applies to wideband signals; it can not handle narrowband signals or an unmodulated carrier. (2) It requires a relatively large number of adaptive loops.
To summarize, with the exception of the patent application by Kenneth Rilling, the prior art is limited. It either does not address the multipath problem, it applies to a very limited range of signal classifications, its approach to the problem is complex, or it requires special tones or codes in the transmitted signal. And consequently, with the exception of the work by Rilling, there is no effective and inexpensive method of removing multipath interference at the communications receiver.